Sweep circuits



Oct- 14, 1941. M. w. BALDWIN, JR 2,259,234

SWEEP CIRCUITS Filed Aug. 26, 1957 F/G/ l v 40 4/ 25 (.36 X l A lvl'VAVAVAVA ,I .30 I /6 l consu/vr I cams/vr L :ounce or l DEV/CEsrNcnaN/z/Nr: l0 /4 s/s/vALs 5.3i l 4 I T J-/s /lz sll l ,//7 l TL /3 40l"\ /V/ caNsrANr Y CURRENT asv/cs l if -FQZ *250 A C' F/a TIME VOLTAGEVOL TAGE DROP I ACROSS SWEEF' N CONDENSERS "1 Q IN1/wrm By M. W. BALDW/MJR.

LQf-UNI'TED 'i STAT Patented Oct. 14, 1941 Es eATEN-Ti oFFIcEf-f Y SWEEPCIRCUITS Millard W. Baldwin, Jr., Glen RidgeLfN.V JQ, as-l signor toBell Telephone Laboratories, Incorporated, New York, N

' York .'JY.,V a'corporation ofNew Application August 2c, 1937, serialNo. 160,992 y 16 claims. Aloll.. 250;;27) e .rfaThis application relatesto electric condenser "charging and dischargingv circuits and more par'-fticularly to sweep circuits such as those used in 'television scanning.

@In Patent 2,180,365 issued November 21, 1939 K,

vztoll. R-Norton, there is disclosed a sweep circuit 4.forJgeneratingsaw-toothed voltage wave forms suitable for television scanning, whichvoltages anode-of a-cathode ray tube. In this Norton 'circuit -two'sweepcondensersin series with their 'common terminal connected to ground are"ich'arged through constant current devices substantially linearly withrespect to time, these c.charges being suddenly removed when a pair oigas-filled grid controlled tubes are periodically and simultaneouslydischarged in accordance ,withthe reception of synchronizing impulses.The sweep condensers are connected respectively tothe plates of a pairofdeflection plates in the 'cathode ray tube by means of couplingresistances vandvcoupling condensers. Some coupling means farenecessaryin the Norton circuit because the voltage of the v'outside terminal ofone of the sweep condensers is-always positive with respect to that ofthe common terminal and that of .the fothe'rf outside terminal isalways'negative with respect thereto, while the voltages kof there-,spective deflection plates of the cathode ray tube 'mustfswingbetweenpositive and negative values z 4withrrespect to a fixed orneutral potential, that of the anode, ifv the .beam is to be deflectedYon both sides of its neutraler rest position.

*iIt isfar'robject of this invention to provide kan improved balancedsweep circuit in which'the sweep condensers are directly coupled'to theop- .aposite plates of'a pair of deflection plates ina cathode ray tube.l

:.:sIt is another object toprovide a balanced sweep lvcircuitthaving theadvantage over .the above- 'mentioned sweep circuit that the voltage ofthe source providing the charging current for Lthe sweep `condensers isreduced by a factor of two.

Itis afurther object'of this invention to provide a sweep circuitin'which apair of sweep condensers connected-in series with their commonterminal fixed 'at ground or other potential have the charges thereonvaried in sucha mannerthat the potential of each outside terminalwith'resp'ect to the xed potential'of the common terminal isvalternately positive and'negative, andv the sum of the'separatelpotential differences between each ungrounded terminal and. `ground issubstantially zero at every instant.

'r'naccorda'nce with the invention, the common .terminal of the: twosweep condensers is maintained at thelsame reference potential.

During a portion ofthe cycle the vpotential of the .outside vterminal ofone condenser is caused to change `substantially.uniformly to morepositive values =by means of current from a suitabledirect cur- Vrent.source acting. through an appropriate kcurvare 4balanced with respecttothe potential of Van rent limiting device, and the potential of theout- .side terminal ofthe secondcondenser is .caused to .changesubstantially uniformly and at the ksame .rate to more negativevalues-by ,means of. cur.- -rent from a similar `source'acting througha cur-'.rentlimiting device..V .Upon the reception of synchronizing impulses,two gas-filled grid controlled electron dischargedevices becomeconducting and so establish' momentary cross-connections. be- `tween.each outside .condenser terminal `and that one of the direct currentsources which initially :charges the opposite condenser, thereby.reversing `.quickly the polarity of each of the outside con- .:denserterminals, :whereupon the discharge de- ,.vice's#becomenonfconductingand the cycle is repeated. By suitably `proportioning theV circuitelements the potential of each condenser terminal .with resp ect toground or reference potential canbey vmade to average substantially zeroover the cycle.

VTheoutput of each condenser may thus Vbe made a substantially'vunbiased voltage wave. In the circuit described in the above-mentionedNorton patent it was found that' ittakes a matter `of tento fifteenseconds for the couplingv condensers to become initially charged andtheelectronsgbea'mV of the cathode ray tube isnot, of course, deiiectedsymmetrically until these condensers are charged. By eliminating theAcoupling condensers this `disadvantage is avoided. Moreover, 'thecoupling condensers are costlybecause of the requirement that theirleakage be extremely low. 1

, anced sweep circuit, which maybe used, for exf ample, as'a' suitable.source to generate-saw-tooth voltages' tobe applied preferably acrossonegpair 'of deflecting plates in a cathode ray tubetobe t used intelevision scanning to cause deection of the cathode ray beam in onedirection, as, for eX- ample, in a horizontal direction. A similarcircuit, differing only from this one in the value of the circuitconstants, may be used to produce vertical deilection. This circuitcomprises sweep condensers I and II having their common terminal I2connected to ground and to the anode I3 of a cathode ray tube I4, withmeans for charging and discharging these condensers simultaneously.

Considering the voltage relations indicated in Fig. 3, and starting withthe condition represented at point E, condenserl I0 is slowly andlinearly charged to a positive potential indicated by the point A, whichwill be designated potential +En, by means of the source of directcurrent I5, which may, for example, be 250 volts, acting through theconstant current device I6.

Preferably the device I6 comprises a pentode having a cathode, an anode,a control grid, a screen grid and a suppressor grid, with these grids soconnected with respect to the cathode and the anode that any decrease involtage of the plate circuit of the device I6 caused by the changedcharge on the condenser ID tending to reduce the plate current willdecrease the bias of one or more of the grids and tend to keep thecurrent through the tube at a constant value. Thus the rate of charge ofthe condenser I0 tends to remain constant. For a more completedescription of the method of operation of pentode tubes having feedbackcircuits on one or more of their Ygrids to produce a constant currentdevice for use in condenser charging or discharging circuits, referencemaybe made to Patent 2,180,364 issued November 21, 1939 to F. R. Norton.1

In a lsimilar manner the condenser II is charged by means of a source ofdirect current I1, which is similar to the directl current source I5,through a constant current device I8 which is similar to the constantcurrent device I6, Thus, after a certain period of time, point Y will beplaced at a potential in the negative direction represented by point Bin Fig. 3, this potential being designated El). Thus, after the periodof time represented by the distance EF, Fig. 3, the potential differencebetween Ypoints X and Y reaches a value off2Eo.

A suitable circuit for suddenly reversing the charges on the condensersI0 and Ilat periodic intervals comprises a source of synchronizingsignals or impulses 20 connected to the primary winding 2l of atransformer 22 which has two secondary windings 23Yand 24 connnectedrespectively `to the grids 25 and 20Y Vof gas-filled grid-controlledelectron discharge devices T1 and T2 through potentiometers 2'! and V28.

The source of synchronizing signals or impulses for controlling thedischarge of the sweep condensers may be located at a transmitterstation or may be local to the circuit controlled by it and may take anyof a variety of well-known forms. For example, if the circuit now beingdescribed were used in connection Awith a cathode ray tube at atelevision receiving station, and a scanning disc were use d to generatethe image andY synchronizing signals, the arrangement for generating thesynchronizing signalssirnilar to the one described in Patent 2,050,363,August 11, 1936 to E. R.. Morton may be used. Or, if a cathode ray tubeis used at the television transmitting station, pulses, generated by thebeam, or otherwise, before or after orduring the return of the beamafter its maximum horizontal and also after its maximum verticaldeflection, assuming horizontal line scanning, may for example be usedto generate the synchronizing signals for the corresponding sweepcircuits at the receiving station.

When a scanning disc is used to generate the synchronizing signals orimpulses, the transformer 22 is preferably of the type which isgenerally known as an impulse transformer. This transformer has a highpermeability core 29 of very small cross-sectional area. The purpose ofthe impulse transformer is to generate sharp voltage pulses in order todischarge the gas-filled grid-controlled electron discharge devices atthe correct times. By this means, random variations in wave amplitudebetween successive cycles of the synchronizing signals caused byimperfections in the signal generating means are reduced. When anapproximately sinusoidal current is passed through the primary winding2| of the impulse transformer 22, the flux through the core 29 changesslowly during most of the cycle due to saturation but it changes veryrapidly when the current is reversed. This rapid change in flux producesa sharp voltage pulse across the secondary vvi-ndings 23 and 24 whichhas more nearly the desired relation to the position of the scanningaperture in each successive cycle of the synchron'ming signals orimpulses than the voltages obtained from an ordinary transformer wouldhave. For a more complete description of impulse transformers per se, ofthe type just mentioned, reference may be made to Patent 1,936,153 to E.T. Burton, issued November 21,

The gas-lilled grid-controlled electron discharge devices T1 and T2 arepreferably of the three-electrode hot-cathode type generally known asthyratrons comprising respectively a suitable envelope enclosing acathode 30, an anode 3l and a grid 25 for T1 and a cathode 32, an anode33, and a grid 26 for T2. Suitable sources of direct current biasingpotential, such as batteries 34 and 35, complete the grid circuits ofthe tubes T1 and T2, respectively, and resistances 36 and 31 are placedin the plate circuits of the tubes T1 and T2, respectively, in order tolimit the peak discharge current through these tubes. The plate circuitsof the tubes T1 and T2 are connected in a series circuit starting withresistance 3S and thence through the sweep condensers I0 and II, thedischarge path of tube T2, the resistance 3l, the sources of directcurrent I5 and II, and the discharge path of tube T1 back to theresistance 3S. condensers 50 and 5I are connected in shunt with thesources I5 and II, respectively, to bypass the current changes aroundthese sources, although these condensers are not essential. Where' eachsource is a rectifier and 'lterg the lter circuit usually includes acondenser so no additional condenser is required. EvenY if batteries areused, the condensers are not absolutely necessary buttheir use isadvisable. By means of this arrangement the voltage difference betweenpoints X and Y caused by the charge on condensers IIJ and II isdissipated between the tubes T1 and T2 in series rather than througheither of these two alone, thus allowing a higher voltage sweep circuitthan would be possible with one gas-lled grid-controlled tube actingalone to remove the charge on the condensers I0 and II.

When the cathode of the tube T1 is connected to the negative side of thesource of potential I'I and? the anode o'f the ltube' T2 is'. connectedthrough the resistance. 3l :to the positive .side ,of th source' ofpotential I5', when discharges .occur inthe tubes :Trand T2`, thepotential. .of the point Xfislbioughtsudde'nly to approximately `thepotetialrepresentedby the point'B, Fig.-Y v3, while the potential of thepoint Y is brought suddenly to thepotential indicated .by thepointA..Stated diierent1y,.the potential .of .the point X which Wasn-TEO'.suddenly passes through Zero and becomes'v #Eni and vthe potentialof.thexpoint Y Whichwas -Eo..suddenly passes through zero andxbecomes +En.The vcycle vbegins again and thefzpotential of point X .is slowlyandlinearly causedqto change its potential from fEo (pointcircuit'corresponding to the .point X in Fig.. 1

would-,yary in potential Vfrom zero to a positive value.;.designated,askAi andv be suddenly reduced rrtoez-zer o,1r -at fpoint F1, at whichpoint the cycle would-startagaim after which the ypotential of othisontiwould slowlyand: linearly attain the potential C1., ,Thechargeslwould lthen be quickly removedagain yand the cycle of slowincreases. in

DQtental and abrupt; decreases would takerplace c :yclically. l In orderto make the'change in volt; age across the condenser corresponding' tocondenser Illequalto 2Etas in the vcase of the sweep circuit of thisapplication, it would be necessary for the voltage of the chargingbattery to be approximately twice that of` the source used in the sweepcircuit of this invention.

eferring again to Fig.,2, in a circuit of the e of that disclosedtin theNorton Patent No.

2,180,365, the potential of a point corresponding tothe point Yin Fig.lhwould change from zero tog-Ziiirepresented by point B1, return to zeroftgpolt Fi-vsuddenly upon the application of an impulse, and then returnslowly toa potential of TT'Zlib at point D1. l

1 @will benoted, therefore, in comparing the sweeovoltages representedby Fig. ,2 and Fig. 3,

, that due to thefact that the potentials of the pointsfrX and Y1 in thecircuit of this invention swing from oneside of the reference potentialto an equal Value on .the -other, which value is determined kbytheiVoltage source, the difference dfromthe.reference potential at thepoint vof max-Y imumchargein the cycle is required to be only one-halfwhat it was in the Norton circuit to pro-l duce, exactly the same amountof voltage swing.

further.vv advantage of thisV circuit lis that the co pling .Vcondenserswhich are used in .the Nor- Y ton'circuitto allow the potentialofeachsweep plateto swing togpotentials on each side Vof the ieierencepoint are not necessary, inasmuch as the potentials generatedl at thepoints'X and Y swing'. in the Amanner desired. Thus, the external oroutside terminals of the'condensers Il! and IIfmaybe connecteddirectly-to the deflecting 'plates' 4t .ana-4| ofthe cathode ray tubeI4.

Thelcornplete operation of the circuit shown in 'Flgl lfis `as follows:Let itbe assumed that condensers Ik and II are fully discharged and thestartingipoint of thecycle is atpoint E in Fig. 3. pointf-,condensers II3,v and .I I v are VVslowly chargedat a substantially; linear rate bymeans 0f? the 1potental sources'l5 and I'I and Athe constantlcurrentdevices I6 and I8, PointX thus attains .a potential Eo at'point A andpoint yY attains a potential Eo at point BV which potential isYsubstantially equal Y and Opposite to that of y X, the common terminalpotential being the reference potential. The sweep plates l0/and -4Ibeing directly Yconnected to the points X and Y, of course follow thepotentials of these points exactly. At periodic intervals, as forexfarnpleiat 'the end of each scanning line in the situation where thesweep circuit is used for horizontal scanning, a synchronizing signaliorv pulse is generated bythe source and caused tojenergize the primarywinding 2| of the trans-.- former 22. Because of the high permeabilityof the core 29 and the restricted cross-'section'thereof sharp Voltagepulses are induced inv the, sec*- ondary lwindings 23 and 24 thepositive portions of which Vare sufficient to overcome the negative biason the tubes Tr and T2, causing these tubes togdischarge theucondensersI0 and Il simultaneously and immediately and suddenly charge them. in,the opposite direction. This quickly brings the anode of each of thegasffilled tubes T1 land T2 to a potential with ,respect to Aits.respective cathode which is Abelowgtlie/potential sufficient to maintainionization int'fthese tubes and they become non-conducting. Thecondensers III and II are then linearly-discharged through the constantcurrent devices I6 and I8, respectively, until the potentials of X and Yreach zero.r Continuing the cycle, the condensers I0 and II are thenlinearly charged through these constant current devices until thepotentials of X and Y reach -l-Ep and -Eo, respectively, as in therststage described above. charge and discharge of the condensers I0 and I Iproduces across the points X and Y a saw-tooth voltagewave at theperiodicity of the eiective synchronizing pulses, which Wave is directlyappliedlacross the plates 40 and 4I;v The potentials applied to theplates areV balanced with respect to the potential of the anode I3 thuseliminating resulted had the sweepy circuit been unbalanced.

' While this invention lhasbeen illustrated and described as a circuitfor lcausing the potentials to be balanced with respect Vto thepotential of an .anode of a cathode ray tube which is lplaced atV groundpotential (the potential of the coin-f mon terminal of the twoV sweepcondensers), it is obvioiisthat the invention is applicable 'also wherethe anode is' placed at a potential above ground as, for example, 5000Volts. In such a situation, the common terminal .may .be placed at thispotential above ground inwhich case. direct coupling'is usedas is shownin Fig. 1. -Alternately, the common terminal is left at ground potentialand coupling condensers of the order of '0.1 fnfd. for a 24cycle'frequency are used between the pointsX and 4I) and thelpoints Yand. 4I respectively to insulate the low voltage The cycle of are used.In general, it is preferred that the leakage resistance of the couplingcondensers be much higher than the leakage resistance of the sweepcondensers (although this, too, should be high). The total leakageresistance of each sweep condenser and the corresponding couplingcondenser should be at least as high as 100 megohms and is preferably20,000 or 30,000 megohms. In order to have a substantially linear sweepvoltage, the product of the capacity of one coupling condenser in faradsand the resistance of one half the coupling resistance should be fromfifty to one hundred times the tim-e required to make a complete sweep.For a circuit in which coupling condensers and resistances are used toconnect a sweep circuit to a pair of deflecting plates in a cathode raytube, reference may be made to Patent 2,178,464, issued October 31, 1939to M. W. Baldwin, Jr.

Various other modications may be made without departing from the spiritof the invention. While the invention pertains primarily to sweepcircuits for cathode beams in television systems, it may obviously rindother uses. This is especially true of some of the various aspects ofthe invention, as will be apparent from consideration of the appendedclaims deining the invention in its diierent aspects.

What is claimed is:

1. The combination with a condenser, of means including a source ofcontinuously direct potential for slowly discharging a charge on saidcondenser and immediately slowly charging it in the reverse direction,means including a second source of continuously direct potential forthen quickly discharging it and reversely charging it with current fromsaid second source, and means for maintaining one terminal of saidcondenser at a fixed potential at all times.

2. In combination, a cathode ray tube having an anode and a pair ofdeflect-,ing plates, a pair of condensers having a common terminal andeach of said condensers having an outside terminal, means for directlyconnecting the common terminal of said condensers to said anode, meansfor connecting the outside terminals of said condensers directly andconductively to the respective plates of said pair of deecting plates tomaintain said outside terminals at the potentials of said respectiveplates, and means for cyclically varying the charge across each of saidcondensers, iirst in one direction and then in the other, whereby thepotential of each of said deflecting plates cyclically varies from apotential which is negative with respect to the potential of said anodeto a potential which is positive with respect thereto.

3. In combination, two condensers having a common terminal and eachhaving an outside terminal, means for charging said condensers so thatthe potentials of the outside terminals are substantially equal inmagnitude and opposite in sign, the potential of the common terminalbeing the reference potential, and means for discharging and rechargingsaid condensers to such a point that the potentials of the outsideterminals reach values on opposite sides of the reference potential fromthe potentials they attained during the charging operation.

4. In combination, a condenser, means for placing one terminal of saidcondenser at a Xed potential, means including a source of continuouslydirect potential for charging said condenser with current from saidsource, and means including a second source of continuously direct Ll apotential for discharging said condenser with current from said secondsource and recharging it to suchr a point that the potential of theterminal which is not at a fixed potential reaches a value on theopposite side of the reference potential from the potential attainedduring the charging operation.

5. In combination, a condenser, means for placing one terminal of saidcondenser at a xed potential, means including a source of continuouslydirect potential for charging said condenser with current from saidsource, and means including a second source of continuously directpotential for discharging said condenser and recharging it with currentfrom said second source to such a point that the potential of theterminal which is not at a xed potential reaches a value on the oppositeside of the reference potential from the potential attained during thecharging operation, said means for discharging and recharging saidcondenser with current from said second source including an electrondischarge device.

6. In combination, a condenser, means for placing one terminal of saidcondenser at a fixed potential, means including a source of continuouslydirect potential for charging said condenser with current from saidsource, and means including a second source of continuously directpotential for discharging said condenser and recharging it with currentfrom said second source to such a point that the potential of theterminal which is not at a fixed potential reaches a value on theopposite side of the reference potential from the potential attainedduring the charging operation, said means for discharging and rechargingsaid condenser with current from said second source including a gas-lledelectron discharge device.

7. In combination, a condenser, means for placing one terminal of saidcondenser at a xed potential, means including a source of continuouslydirect potential for charging said condenser with current from saidsource, and means including a second source of continuously directpotential for discharging said condenser and recharging it with currentfrom said second source to such a point that the potential of theterminal which is not at a xed potential reaches a value on the oppositeside of the reference potential from the potential attained during thecharging operation, said means for discharging and recharging saidcondenser with current from said second source including a gas-lled,gridcontrolled electron discharge device.

8. A sweep circuit comprising tWo condensers having a common terminaland each having an outside terminal, means including two sources ofpotential and two constant current devices for charging said condenserssubstantially linearly with respect to time so that the potentials ofthe outside terminals are substantially equal in magnitude and oppositein sign, the potential of the common terminal being the reference point,and means including two gas-filled grid-controlled electron dischargedevices and said two sources of potential for discharging and rechargingsaid condenser to such a point that the potentials of the outsideterminals reach values on opposite sides of the reference potential fromthe potentials they attained during the charging operation.

9. A sweep circuit comprising a pair of condensers connected in series,the common terminal of which is connected to ground, a constant currentdevice and a source lof potential connected between the outside terminalof each condenser and the common terminal, two gas-filledgrid-controlled electron discharge devices havv ing input and outputcircuits, a transformer having a primary winding and two secondarywindings, means for connecting the secondary windings of saidtransformer respectively to the input circuits of said gas-iilledgrid-controlled electron discharge devices, and means for connecting theoutput circuits of said electron discharge devices in a series circuitincluding said two condensers and said two sources of potential.

10. Means for producing an alternating potential from a direct potentialcomprising a source of direct potential, two condensers in a t seriescircuit, and means for alternately and intermittently connecting saidsource to said circuit and connecting each of said condensers to aportion only of said source in reverse direction.

11. The combination witha pair of condensers having a common terminal,each of said condensers having an outside terminal, of means formaintaining the potential of said common terminal fixed and forcyclically varying the charge across each of said condensers, rst in onedirection and then in the other with respect to said xed potential, acathode ray tube having an anode and a pair of deiiecting plates, andcoun .pling means between said condensers and said tube for, at alltimes, causing said anode to have the same potential as said midpointand said deecting plates to have respectively the same potentials assaid outside terminals.

12.V The combination with a condenser, of means for slowly charging andimmediately rap- .idly discharging said condenser, then rapidlyreversely charging and immediately slowly discharging said condenser,said means comprising a source of continuously direct potentialcontinuously conductively connected to said condenser, a second sourceof continuouslyV direct potential, and means for intermittentlyconnecting said second source in series with said condenser to permitcurrent to ow from said second source to said condenser.

13. The combination with a condenser, of means for slowlyr charging andimmediately rapidly discharging said condenser, then rapidly reverselycharging and immediately slowly discharging said condenser, said meanscomprising a source of continuously direct potential continuouslyconductively connected to said condenser,

a second source of continuously direct potential, means forintermittently connecting said second source in series with saidcondenser to permit current to ow from said second source to saidcondenser, and means including a source of periodically varyingpotential for controlling said means for connecting said second sourceof continuously direct potential in circuit with said condenser.

14. The combination with a condenser, of two parallel circuits connectedto the terminals thereof, a source of continuously direct potential ineach of said circuits, said potentials being poled in oppositedirections with respect to the terminals of the condenser, and means inone of said circuits for effectively periodically rendering said circuitalternately conductive and nonconductive.

15. The combination with a condensenof two parallel circuits connectedto the terminals thereof, a source of continuously direct potential ineach of said circuits, said potentials being poled in oppositedirections with respect to the terminals of the condenser, and means inone of said circuits for effectively periodically rendering saidcircuits alternatively conductive and non-conductive, both of saidcircuits being non-reactive.

16. In combination, a condenser, means for placing one terminal of saidcondenser at a fixed potential, means including a source of continuouslydirect potential for charging said condenser with current from saidsource, said condenser charging means including an impedance element forcontrolling the rate of current ow from said source to said condenser,and means including a second source of continuously direct potential fordischarging said. condenser and recharging it with current from saidsecond source to sucha point that the potential of the terminal which isnot at a fixed potential reaches a value on the opposite side of thereference potential from the potential attained during the chargingoperation, said last-mentioned means including a second impedanceelement in series with said second source and said condenser, and meansfor periodically alternately raising the impedance of said secondimpedance element to substantially block the ow of current therethroughand re- 1 ducing it to permit the ow of current /therethrough.

MILLARD W. BALDWIN, JR.

